Apparatus for sensing the frequency of machining pulses from an edm power supply

ABSTRACT

An apparatus for monitoring the frequency of output pulses from an EDM power supply and terminating the operation of the supply when the output pulse frequency exceeds a predetermined magnitude. The apparatus comprises a frequency sensing circuit that produces an output signal having a magnitude that is a function of the output pulse frequency. A switching network is connected to the sensing circuit and terminates the operation of the power supply in response to an output signal representing the predetermined output pulse frequency magnitude.

Inventor Philip E. Berghausen Cincinnati, Ohio Appl. No. 54,102

Filed July 13, 1970 Patented Nov. 23, 1971 Assignee Cincinnati Milacron lnc.

Cincinnati, Ohio APPARATUS FOR SENSING THE FREQUENCY OF MACHINING PULSES FROM AN EDM POWER SUPPLY [56] References Cited UNITED STATES PATENTS 3,229,158 l/l966 Jensen 331/] 13.1 3,38l,l4l 4/1968 Millon 307/290 3,217,270 ll/l965 Friedrichs et al. 331/172 Primary Examiner-John Kominski Arlorneys- Howard T. Keiser and Jack J. Earl ABSTRACT: An apparatus for monitoring the frequency of output pulses from an EDM power supply and terminating the 2Claims,2 Drawing Figs. operation of the supply when the output pulse frequency ex- U 8 Cl 331 72 ceeds a predetermined magnitude. The apparatus comprises a P 219/69 frequency sensing crrcult that produces an output signal hav- In Cl {323k 9/10 mg a magnitude that IS a function of the output pulse frequen' Fieid 0.. 307/290. cy A swiching network is connected to the sensmg circuil 331/172 S 69 C and terminates the operation of the power supply in response to an output signal representing the predetermined output p pulse frequency magnitude.

SOURCE 2o POWER OSCILLATOR f' L SWITCHING i I 2| CIRCUITS IO 8 FREQUENCY [I 22 SENSING DlSABl-ING CIRCUIT l7 NETWQRK major components:

- frequencies may be BACKGROUND OF THE INVENTION The invention relates generally to an improvement to an electrodischarge machining (EDM) power supply. Specifically, the invention concerns a protection circuit for sensing the frequency of the output pulses from the power supply and extinguishing said output pulses when they exceed a predetermined frequency.

In general, EDM power supplies may be divided into three an oscillator, a direct current source, and power switching circuits. The oscillator, which is a low power source of pulses, is used to drive the power switching circuits which produce output machining pulses by switching the output of the direct current supply. Many traditional power supplies contain an oscillator frequency adjustment that varies the period of the output pulses. In these supplies, the maximum frequency can be readily controlled by limiting the range of the frequency adjustment. More recent power supplies do not have an oscillator frequency adjustment but have N time and OFF time controls. The ON time represents the time duration of an output pulse, and the OFF time represents the time duration between output pulses. With such independent controls, it is possible to obtain very short ON and OFF times and hence very high frequencies. These very high damaging to the power supply switching transistors in the power supply output circuit.

Therefore, applicant proposes a circuit responsive to the oscillator output which produces an output signal that changes as a function of the output pulse frequency of the oscillator. When the frequency is increased to a predetermined level, the output triggers a switching device which is operable to terminate the operation of the power supply. Restoration of the power supply operation requires the attention of an operator who must decrease the supply output frequency by increasing the OFF or ON times.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an EDM power supply illustrating the preferred embodiment ofthe invention.

FIG. 2 is a detailed schematic diagram of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a block diagram illustrating how the frequency measuring circuit operates in combination with a typical EDM power supply. The power supply contains an oscillator 10, which may be a multivibrator or other source of pulses, a direct current source 12, and power switching circuits 14. The power switching circuits 14 are controlled by pulses from the oscillator and switch the output of the source 12 to supply machining pulses to a gap 21 formed between an electrically conductive tool and an electrically conductive workpiece 22. Connected to the output of the oscillator 10 is a frequency sensing circuit 16. The output 17 of the circuit 16 is a signal that changes as a function of the frequency of the output 11 of the oscillator 10. As the oscillator frequency increases, the output 17 also increases. A disabling network 18 is coupled to reached, operates to terminate the output 11 of the oscillator 10.

It should be noted at this point that the exact point chosen to disable the power supply and thereby the power switching circuits is purely an arbitrary one. Since the oscillator is the source of the undesirable condition, i.e. the high frequency, and the device most readily interfaced because of its low power operation, this embodiment illustrates the oscillator 10 as being disabled.

FIG. 2 is a detailed schematic diagram of the invention and related elements. The block 24 is a detailed schematic diagram of the preferred embodiment of the frequency sensing circuit and has an input connected to the output 11 of the oscillator 10. Assume for purposes of this disclosure, the output pulses from the oscillator are negative pulses. During the ON times, the pulse magnitude is at a negative voltage level the input 11 increases, crease as a function thereof. This is readily apparent by looking at the parameters that define the magnitude of the voltage ofthe output 17.

resistor 34 and the capacitor 26.

Further, if the product of the magnitudes of the resistor 34, the capacitor 26, and the frequency of the input I] is very much less than one, then the voltage of the output 17 is a function of the voltage and frequency of the input 11 and the magnitudes of the capacitor 26 and the resistor 34.

The resistor and capacitor sizes and the input voltage magnitude are constant, therefore the magnitude of the output 17 will vary with the frequency ofthe input I].

The preferred embodiment of the switching current 38 is illustrated as a Schmidt trigger comprised of transistors 40, 42

conducting. As mentioned earlier, a predetermined input,

frequencywill result in a voltage magnitude on output 17 to switch transistors 40 and 42 into conduction. This causes. transistor 44 to rapidly terminate conduction. When transistor conductive state.

As mentioned earlier, there are many possible ways to disable the power supply. The oscillator may be disabled internally I at several points, or driver stages of the power switching circuits may be disabled. For reasons of simplicity and ease of explanation, a disabling switching network 50 will be used to shunt to ground the oscillator output 11. When transistors 40 42 start conducting, the base of transistor 46 is driven to a negative level causing conduction through transistor 46 and shunting the oscillator output 11 to ground through the conductor 54. To summarize, when the oscillator frequency exceeds the predetermined magnitude, this frequency is detected by the measuring circuit 24 which produces an output activating the switch 38 and the network 50 thereby shunting the high-frequency pulses on the oscillator output 11 to ground. The power supply is disabled without damage to the power switching circuits. In this case, the power must be turned off in order to release the feedback latch 48 in the Schmidt trigger. During this time, the ON and OFF times must be adjusted to produce a lower frequency. Power is then restored to the power supply, and the device will operate in a normal manner.

While the invention has been illustrated in some detail according to the preferred embodiment shown in the accompanying drawings, and while the preferred illustrated embodiment has been described in some detail, there is no intention to thus limit the invention to such details. On the contrary, it is intended to cover all modifications, alterations and equivalents falling within the spirit and scope of the appended claims.

What is claimed is:

1. In an EDM power supply of the type comprising an oscillator producing pulses having a constant voltage magnitude and a variable frequency, and power switching circuits responsive and sensitive to the frequency of said oscillator pulses for switching an output of a DC source to produce machining pulses from the power supply, an improvement comprising the steps of:

a. sensing the frequency of the oscillator output signal by a frequency sensitive circuit and producing an output signal having a magnitude that is a function of the frequency of the oscillator pulses; and

b. inhibiting by a disabling network the production of pulses from the oscillator in response to a predetermined magnitude of the output signal representing a pulse frequency that may be damaging to the power switching circuits.

2. In an EDM power supply of the type comprising an oscillator producing pulses having a constant voltage magnitude and a variable frequency, and power switching circuits responsive and sensitive to the frequency of said oscillator pulses for switching an output of a DC source to produce machining pulses from the power supply, an improvement comprising:

a. sensing the frequency of the oscillator output signal by a frequency sensitive circuit and producing an output signal having a magnitude that is a function of the frequency of the oscillator pulses;

b. producing by a switching circuit, a second output signal in response to a predetermined magnitude of the first output signal from the sensing circuit representing a pulse frequency that may be damaging to the power switching circuits; and

c. terminating by a disabling network the input of pulses produced by the oscillator to the power switching circuits in response to the second output signal. 

1. In an EDM power supply of the type comprising an oscillator producing pulses having a constant voltage magnitude and a variable frequency, and power switching circuits responsive and sensitive to the frequency of said oscillator pulses for switching an output of a DC source to produce machining pulses from the power supply, an improvement comprising the steps of: a. sensing the frequency of the oscillator output signal by a frequency sensitive circuit and producing an output signal having a magnitude that is a function of the frequency of the oscillator pulses; and b. inhibiting by a disabling network the production of pulses from the oscillator in response to a predetermined magnitude of the output signal representing a pulse frequency that may be damaging to the power switching circuits.
 2. In an EDM power supply of the type comprising an oscillator producing pulses having a constant voltage magnitude and a variable frequency, and power switching circuits responsive and sensitive to the frequency of said oscillator pulses for switching an output of a DC source to produce machining pulses from the power supply, an improvement comprising: a. sensing the frequency of the oscillator output signal by a frequency sensitive circuit and producing an output signal having a magnitude that is a function of the frequency of the oscillator pulses; b. producing by a switching circuit, a second output signal in response to a predetermined magnitude of the first output signal from the sensing circuit representing a pulse frequency that may be damaging to the power switching circuits; and c. terminating by a disabling network the input of pulses prodUced by the oscillator to the power switching circuits in response to the second output signal. 